Amphibious vehicles, hereafter “amphibians”, are frequently steered in their land and marine modes by the same steering wheel. A simple form of amphibian steering is shown in U.S. Pat. No. 5,727,494 (Caserta). In Caserta's proposal, a steering cable is arranged between a rear propulsion unit and the inner shaft of the steering column, which the cable is wound around, and thence back to the marine propulsion unit. Whilst this is a simple arrangement, it is unsuitable for high speed planing amphibians, where the force required to steer the propulsion unit is high. That is, the mechanical advantage of the Caserta arrangement is low. Where an automotive steering column is used, there is a high risk that the steering cable according to Caserta will conflict either with the mechanism designed to ensure crushability of the steering column in an accident; or alternatively, with any telescopic adjustment mechanism designed to ensure an ergonomic driving position.
In another proposal shown in U.S. Pat. No., 5,590,617 (Gere), the linkages between the rear marine propulsion unit and the steering arrangement at the front of the vehicle are bulky and heavy, at a location at the front of the vehicle where weight has to be carefully controlled in a planing amphibian. Furthermore, the steering according to Gere is conformed so as to be operable in either a road mode or a marine mode. To allow this duality, the road steering system depends for operation on the steering rack being held in place by pressurized pneumatic rams. This is somewhat alarming from a safety viewpoint.
It is considered advantageous to have road and marine steering system which can be operated simultaneously. This simplifies control systems, as there are less changes to be made in converting from road mode to marine mode or vice versa. Also, when maneuvering at low speed in water, particularly to direct an amphibian to a slipway, the steering effect of dependent road wheels may be at least as great as that of, for example, a steering nozzle attached to a jet drive. Furthermore, if both systems can be operated together, there is no need for complex systems to ensure that when one or the other system is switched in, it is always initially centered. Finally, there is a safety advantage, in that in the unlikely event of breakage or seizure of the steering cable, a second steering system is available. In this context, it should be noted that the marine steering is self-centering. In the absence of any control input or restraint of movement, passage of water through the jet nozzle will tend to center the nozzle.